1. Field of the Invention:
The present invention relates to a timepiece movement and particularly to a small-sized movement driven by a synchronous motor in a clock such as wall clock, table clock and the like.
2. Description of the Prior Art:
Most of the clocks such as table clocks and wall clocks have been crystal clocks utilizing a crystal oscillator which provided a high accuracy oscillation. Such a crystal oscillator serves as a source of time reference oscillation to secure very well accuracy of time indication.
In such crystal clocks, a movement includes a main mechanism for rotatably driving tim indicating hands by the use of electrical clock pulses. Such a mechanism is generally divided into a synchronous motor section and a gear train section.
The synchronous motor section includes a stator coil to which high accuracy clock pulses obtained by dividing the frequency of crystal oscillation are applied, a stator having stator poles placed under the action of the magnetic flux in the stator coil, and a rotor rotated within the stator poles. On the other hand, the gear train section functions to gradually reduce the velocity of the rotating rotor and to transmit the rotation of the rotor to clock hands.
The external form of the conventional clock movements is normally of substantially circular or square shape for such a purpose that all the parts of the movement should be located about the hour hand of a clock as possible.
If a clock movement has a synchronous motor and a clock gear train arranged about the hour hand, the clock movement can freely be used for either of the wall or table clock. When one or more of external parts such as dial plate, indicating hands and decorative case are changed to other parts for a single type of clock, the latter can be converted into another type of clock.
Since the conventional movements have parts arranged about the hour hand as described, however, the movements are necessarily increased in thickness. If the gear train includes two or three wheel layers, this makes the assembly of the clock difficult in addition to the increased thickness of the movement.
The conventional movements have a further problem in that they cannot sufficiently deal with a certain design of clock. Such a design of clock includes, for example, a clock having a concealed movement. Such a clock has a mechanism for driving time indicator hands which mechanism cannot easily be viewed. This provides a unique design for the clock. Probably, such a unique design will make a new genre for clock design. However, it is extremely difficult to completely conceal all the parts arranged about the hour hand of the clock.
There has been proposed a skeleton clock which is intended to conceal its movement as possible. However, this also could not provide a movement which was satisfactorily concealed.
There is also a proposal to provide a small-sized movement suitable for use in a concealed movement type of clock. In such a case, the small-sized movement must provide a satisfactorily large driving torque. The concealed movement type clock requires time indicator hands which are sufficiently large in comparison with the small size of the movement. This intends to give a question to a user in that such large and long hands cannot be driven only by a mechanism assembled into any one strut or column. Such a question will make the user to recognize a new design for the clock.
On the other hand, however, the synchronous motor must provide an increased torque for driving the large and long hands in the small-sized movement of the clock. In the prior art, such an increased torque was accomplished by increasing the magnetic flux in the rotor. The increase of the torque causes a magnetic connection between the rotor and any metallic part located near the rotor. Particularly, when the small-sized movement has a battery as a source of current supply arranged near the rotor, the latter is magnetically attracted toward the battery to create an unnecessary inclination in the rotor which will cause various malfunctions.
With the small-sized clock movement suitable for use in concealed movement type clocks and capable of driving the large and long hands, therefore, the positional relationship between the rotor and the battery is a very important factor.
Further, the clock movement may include an analog display section to be driven by the synchronous motor. The synchronous motor receives a given drive signal (normally, a fixed cycle pulse signal) from a drive section. The drive section includes a motor drive circuit and a battery.
The analog display section is assembled with the drive section to form a unitary clock movement. However, such a unitary clock movement tends to increase the whole size of the movement and is particularly disadvantageous for the concealed movement type clocks as aforementioned.